ZSM-12 and its conventional preparation in the presence of a tetramethylammonium or tetraethylammonium directing agent are taught by U.S. Pat. No. 3,832,449, the entire disclosure of which is incorporated herein by reference. ZSM-12 has a distinctive X-ray diffraction pattern which distinguishes it from other known crystalline materials.
There is a growing demand for ZSM-12-type catalyst materials in the chemical and refining fields. As a result there is significant interest in developing improved techniques for the synthesis of ZSM-12.
U.S. Pat. No. 4,391,785 discloses a method for the synthesis of ZSM-12 from a reaction mixture comprising, as a directing agent, a compound selected from the group consisting of a dimethylpyridinium halide and a dimethylpyrrolidinium halide.
U.S. Pat. Nos. 4,452,769 and 4,537,758 disclose methods for synthesizing ZSM-12 from a reaction mixture containing methyltriethylammonium ions as the directing agent. These patents are primarily directed to producing high SiO2/Al2O3 ratio forms of ZSM-12, greater than 80 in the case of the '769 patent and greater than 200 in the case of the '758 patent. Moreover, one of the stated advantages in the '769 patent of using methyltriethylammonium ions as the directing agent is the production of large crystal size materials.
Other organic directing agents that have been used to synthesize ZSM-12 include DABCO-Cn-diquat ions where n=4, 5, 6 or 10 (see U.S. Pat. No. 4,482,531), bis(dimethylpiperidinium)trimethylene ions (see U.S. Pat. No. 4,539,193), benzyltriethylammonium ions (see U.S. Pat. No. 4,552,738), dibenzyldiethylammonium ions (see EP-A-167,232), dimethyldiethylammonium ions (see U.S. Pat. No. 4,552,739), benzyltrimethylammonium ions (see U.S. Pat. No. 4,585,637), bis(N-methylpyridyl)ethylinium ions (see U.S. Pat. No. 4,585,746), hexamethyleneimine (U.S. Pat. No. 5,021,141), and decamethonium ions (see U.S. Pat. No. 5,192,521) bis(methylpyrrolidinium) diquat-n ions where n=4, 5, or 6.
Although influenced by variables such as the silica/alumina molar ratio of the reaction mixture, temperature and stirring, the crystal morphology of synthetic zeolites, such as ZSM-12, is mainly dominated by the choice of directing agent used in the crystallization. For example, in the case of ZSM-12, needle-shaped crystals can be produced using a benzyltrimethylammonium directing agent, rice-shaped crystals can be made in the presence of tetraethylammonium salts, and bundles of hexagonal platelets can be prepared from a hexamethyleneimine directing agent. The control of zeolite crystal morphology is very important from the standpoint of activity and stability enhancement. For most catalytic applications, small crystal size is desirable for high activity and stability because of the higher surface area, and hence the shorter diffusion path, of small crystal materials.
For example, U.S. Pat. No. 6,893,624 discloses the synthesis of ZSM-12 having a silica to alumina molar ratio less than 60, an average crystal size of the material is less than 0.1 micron and a Diffusion Parameter for mesitylene of at least 1000×10−6 sec−1 when measured at a temperature of 100° C. and a mesitylene pressure of 2 torr. The synthesis is conducted by crystallizing a reaction mixture comprising a source of an oxide of a trivalent element X, a source of an oxide of a tetravalent element Y, methyltriethylammonium cations (R) as a template, an alkali metal and/or alkaline earth metal ion source M having the valency n and water; wherein the mixture has the following composition in terms of the molar ratios: YO2/X2O3 is 40 to 80, H2O/YO2 is 15 to 40, OH−/YO2 is 0.15 to 0.4, M/YO2 is 0.15 to 0.4 and R/YO2 is 0.15 to 0.4. The crystallization is carried out at a temperature of 170° C. or less for a time of about 50 to 500 hr.
In addition, US Patent Application Publication No. 2008/0035525 discloses a process for producing ZSM-12 having a primary crystal size of <0.1 μm; and a specific volume, determined by mercury porosimetry at a maximum pressure of 4000 bar, of 30-200 mm3/g in a pore radius range of 4-10 nm. The process involves crystallization of a synthesis gel composition comprising an aluminum source, precipitated silica as a silicon source, TEA+ as a template, an alkali metal and/or alkaline earth metal ion source M having the valency n and water; in which the molar H2O:SiO2 ratio is selected between 5 and 15, the molar M2/nO:SiO2 ratio is within the range from 0.01 to 0.045, molar TEA+/SiO2 ratio is between about 0.10 and 0.18, the molar SiO2/Al2O3 ratio is within a range from 50 to 150. The crystallization is carried out at a temperature of from about 120 to 200° C., preferably from about 140 to 180° C., for a time of about 50 to 500 hr, in particular from about 100 to 250 hr.
An object of the present invention is to provide a small crystal, high activity form of ZSM-12 which exhibits enhanced activity in the conversion of C9+ aromatic hydrocarbons to xylenes.
It is to be appreciated that, although ZSM-12 is normally synthesized as an aluminosilicate, the framework aluminum can be partially or completely replaced by other trivalent elements, such as boron and/or iron and/or gallium, and the framework silicon can be partially or completely replaced by other tetravalent elements such as germanium.